Traditional turbine engine ignition systems utilize a high energy capacitive discharge circuit which provides an ignition spark that typically delivers between one and five joules of energy at a rate of roughly ten sparks per second. These relatively high amounts of spark energy are needed to ignite the jet fuel, which by design has a relatively low flammability that is closer to kerosene than gasoline. As the turbine begins to spin and the fuel enters the system, the air/fuel ratio moves through a window in which the ratio is optimal for ignition. That is, the air/fuel ratio changes from being too lean for ignition to being too rich, and it is at the cross-over between these two states that conditions are optimal for ignition. In the typical large turbine engine, the air/fuel mixture moves through this window rather slowly. Accordingly, the relatively low spark rate (e.g., ten sparks per second) provided by the traditional capacitive discharge ignition systems is suitable for catching the air/fuel mixture within this window.
Apart from capacitive discharge circuits, various inductive ignition circuits have also been proposed for turbine and internal combustion engines. These systems generally utilize a transformer or other inductive device to store energy used in generating the spark. See, for example, U.S. Pat. No. 5,139,004, issued Aug. 18, 1992 to M.W. Gose et al., which discloses an inductive ignition circuit for an internal combustion engine. The ignition circuit utilizes a drive transistor to control current flow through the primary of a step-up transformer. The drive transistor is switched on and off in synchronism with rotation of the engine's crankshaft. A resistor in series with the primary winding and drive transistor is used to sense current through the primary and is connected to the transistor's drive circuit to bias the drive transistor into a current-limiting mode when the primary winding current increases to a predetermined level. The drive circuit includes an RC timing circuit that is used to prevent the drive transistor from being biased back on by spurious noise prior to the succeeding timing pulse from the crankshaft's position sensor. The signal from this timing circuit is provided to a comparator circuit along with a reference voltage and the comparator output is used to hold the drive transistor off until the signal from the timing circuit falls below the reference voltage.
Another such inductive ignition circuit is disclosed in U.S. Pat. No. 4,738,239, issued Apr. 19, 1988 to D. L. Haines et al. The circuit includes a high side connected drive transistor that is switched on and off by a signal generator. The transistor is turned off by switching its gate to ground. During flyback of the transformer, the voltage at the transistor's source is driven negative. To prevent the transistor from switching back on, a separate transistor is used to clamp the gate of the transistor to its source during flyback of the transformer. As with the Gose et al. circuit, the spark rate is determined based on crankshaft position.
Ignition circuits that do not utilize flyback for spark generation have also been utilized. See, for example, U.S. Pat. No. 5,587,630, issued Dec. 24, 1996 to K. A. Dooley, which discloses a continuous plasma ignition system that utilizes an LC resonant circuit operating at between 10-30 KHz. The circuit includes a transformer and drive transistor which is switched either by a timer circuit having a frequency that is set by an RC circuit or by closed loop feedback from the transformer secondary using a voltage controlled oscillator to drive the circuit towards resonance. U.S. Pat. No. 4,918,569, issued Apr. 17, 1990 to T. Maeda et al., discloses a forward type ignition circuit having a high self-resonance frequency which provides a high voltage output with a short rise time. The drive circuit includes a transformer and a transistor for switching current through the transformer primary. A sense resistor in the ground path of the secondary provides a detection signal which is fed to a control circuit that switches off the drive transistor when the current through the secondary becomes sufficiently high.
Various hybrid ignition systems have been proposed in which an inductive storage device is used in combination with a transformer or capacitor to provide the spark energy. For example, U.S. Pat. No. 5,065,073, issued Nov. 12, 1991 to J. R. Frus, discloses a capacitive discharge ignition circuit which includes a dc-dc converter having a flyback transformer that is used to charge the circuit's main storage capacitor. The dc-dc converter uses a feedback winding which supplies positive bias to its drive transistor during turn-on of the transistor. A sense resistor in the primary winding current path is used to initially switch the transistor back off once the current through the primary gets sufficiently high. Thereafter, flyback energy from the feedback winding provides negative bias to hold the drive transistor off during flyback. Spark rate control is provided by way of a separate timing circuit that provides a disable signal to the drive transistor to maintain it in an off state for a period of time after flyback of the transformer.
The foregoing ignition circuits have been designed primarily for use in automotive internal combustion engines and in aircraft turbine engines. More recently, however, smaller turbine systems that are powered by natural gas and other nontraditional fuel sources have begun to appear. Not only can these systems be ignited with less spark energy than that supplied by traditional capacitive discharge ignition systems, but also they may move through their optimal air/fuel mixture window very quickly, especially in micro-turbine systems such as are sometimes used in electric generators. Consequently, the traditional capacitive discharge ignition systems can be too slow to provide optimal ignition of the turbine system. While some of the ignition systems described above can achieve the necessary spark rates and, in the case of the Dooley system, can provide a continuous plasma arc, most of these systems do not provide closed-loop spark rate control that is selectable over a wide range.
Accordingly, it is an object of the invention to provide a low-cost inductive ignition circuit that provides reliable ignition of the newer types of small turbine engines such as micro-turbines used in electric generators.